Mice homozygous for either the lpr or gld genes develop phenotypically identical autoimmune disorders. The gene responsible for the pathology in lpr/lpr mice encodes the Fas antigen, a protein associated with the induction of programmed cell death. To determine if the defect associated with gld represents a mutation in the ligand for Fas, we have assessed the ability of lymphoid cells from homozygous gld/gld mice to lyse target cells in a Fas-dependent manner. Using an antagonistic antibody to Fas, we demonstrate that activated T cells from normal and lpr mice are capable of inducing Fas-mediated lysis of tumor target cells. In contrast, activated T cells from gld/gld mice fail to induce lysis of tumor targets, although cells from gld mice are able to lyse specific allogeneic targets following mixed lymphocyte culture. In addition, activated T cells from gld/gld homozygous animals are not capable of binding to a Fas.Fc fusion protein at high levels, whereas activated T cells from normal and lpr/lpr animals bind Fas.Fc efficiently. These data indicate that mice homozygous for gld are unable to express a functional ligand for Fas.